Fundamentals of Networking

1
Fundamentals of Networking Telecommunications

• Class 9
• Local Area Networking (LAN)
• Technology

2
LAN Applications

• Personal Computer LANs
• Low cost
• Relatively limited data rate
• Back end networks and storage area networks
• Interconnecting large systems (e.g. --
  mainframes)
• High speed interfaces high aggregate data rates
• Distributed access over limited distance number
  of devices
• High speed office networks
• Desktop image processing (e.g. animation)
• High capacity local storage
• Backbone LANs
• Interconnect low speed local LANs
• Reliability, Capacity, Cost are driving factors

3
LAN Architecture Considerations

• Protocol architecture
• Media access control
• Logical Link Control
• Topologies

4
LAN Protocol Architecture

• Lower layers of OSI model
• Key Architecture the IEEE 802 reference model
• Physical Layer
• Logical link control (LLC) sublayer
• Media access control (MAC) sublayer

5
IEEE 802.x vs. OSI Models
6
IEEE 802 Physical Layer

• Physical Layer considerations
• Physical Medium (fiber, copper, etc.)
• Encoding/decoding
• Preamble generation/removal
• Bit transmission/reception
• Transmission medium and topology
• More discussion of this throughout later slides

7
IEEE 802 Layers - MAC LLC

• MAC sublayer functions
• Assembly of data into frame with address and
  error detection fields
• Disassembly of frame at destination
• Address recognition
• Error detection
• Govern access to transmission medium
• Not found in traditional layer 2 data link
  control
• For the same LLC, several MAC options may be
  available
• Logical Link Control (LLC) functions
• Interface to higher levels
• Flow and error control

8
LAN Protocols in Context
9
Medium Access Control Characteristics

• Where?
• Central
• Greater control
• Simple access logic at station
• Avoids problems of co-ordination
• Single point of failure Potential bottleneck
• Distributed
• How?
• Synchronous
• Specific capacity dedicated to connection
• Asynchronous
• In response to demand

10
MAC in Asynchronous Systems

• Round robin
• Good if many stations have data to transmit over
  extended period(s) of time
• Reservation
• Good for stream-oriented traffic
• Contention
• Good for bursty traffic
• All stations contend for time
• Distributed mechanism
• Relatively simple to implement
• Efficient under moderate load
• Tends to collapse under heavy load

11
MAC Frame Format

• MAC layer receives data from LLC layer
• General Frame Format
• MAC control
• Destination MAC address
• Source MAC address
• LLC Payload (Data)
• Cyclic Redundancy Check (CRC)
• MAC layer detects errors and discards frames
• LLC optionally retransmits unsuccessful frames

12
Logical Link Control (LLC)

• Transmission of link level PDUs between two
  stations
• Must support multi-access, shared medium
• Relieved of some link access details by MAC layer
• Design allows the same LLC specs across multiple
  MAC/PHY standards
• Addressing involves specifying source and
  destination LLC users
• Referred to as service access points (SAP)
• Specific addresses typically assigned to upper
  layer protocols

13
LLC Services

• Based on services defined in HDLC
• Three different LLC services
• Unacknowledged connectionless service
• Connection mode service
• Acknowledged connectionless service

14
LLC Protocol

• Also modeled after HDLC with exceptions
• Uses Asynchronous Balanced Mode only to support
  connection mode service (type 2 operation)
• Supports unacknowledged connectionless service
  with UI PDUs (type 1 operation)
• Supports acknowledged connectionless service with
  two new PDUs (type 3 operation)
• Allows multiplexing by the use of LLC SAPs
  (addresses)
• For LAN operation, Type 1 operation is used
• No acknowledgement, error, or flow control
• Error detection frame discard used, higher
  layers must provide additional functionality

15
Typical LLC/MAC Frame Format
16
Basic LAN Topologies
The way in which networked stations are
interconnected
17
Bus and Tree Topologies

• Multipoint medium
• Transmission propagates throughout medium
• Heard by all stations
• Need to identify target station (unique
  addresses)
• Full duplex connection between station and tap
• Allows for transmission and reception
• Need to regulate packet transmission
• To avoid collisions
• To avoid one station hogging the medium
• Terminator absorbs frames at end of medium

18
Frame Transmission - Bus LAN
19
Bus/Tree LAN Signal Balancing

• Another important consideration -- signal must
  meet receivers minimum signal strength
  requirements throughout LAN
• Give adequate signal to noise ratio (SNR)
• Cannot be so strong that it overloads transmitter
  for close stations
• Must satisfy these constraints for all
  combinations of sending receiving stations
  (signal balancing)
• Because of this (among other reasons), it is
  desirable to divide network into small segments
• Link segments with repeaters, bridges, or routers

20
Bus LAN Transmission Media

• Twisted pair
• Not practical in shared bus at higher data rates
• Baseband coaxial cable
• Used by Ethernet
• Broadband coaxial cable
• Included in 802.3 specification but no longer
  made
• Optical fiber
• Expensive
• Difficulty with bus architecture
• Not used
• Few new installations
• Replaced by star based twisted pair and optical
  fiber

21
Example Baseband Coaxial Cable

• Uses digital signaling
• Typically Manchester encoding
• Entire frequency spectrum of cable used
• Single shared channel on cable
• Bi-directional transmission
• Few kilometer maximum range
• Most widespread use is for Ethernet (basis for
  IEEE 802.3) at 10Mbps
• Uses 50 ohm cable (impedance) like RG-58

22
Ring Topology

• Repeaters joined by point to point links in
  closed loop
• Receive data on one link and retransmit on
  another
• Links are unidirectional
• Stations attach to repeaters
• Data is passed in frames
• Circulate past all stations
• Destination recognizes address and copies frame
• Frame circulates back to source where it is
  removed
• Medium access control mechanism determines when
  station can insert frame

23
Ring-based LANs

• Each repeater connects to two others via
  unidirectional transmission links
• Data transferred bit by bit from one repeater to
  the next
• Repeater regenerates and retransmits each bit
• Repeater performs data insertion, data reception,
  data removal
• Repeater acts as attachment point
• Packet removed by transmitter after one trip
  round ring

24
Frame Transmission - Ring LAN
25
Ring Repeater States
26
Ring Listen State Functions

• Scan passing bit stream for patterns
• Address of attached station
• Broadcast address
• Token (permission to transmit) or control frames
• Copy incoming bit and send to attached station
• While simultaneously forwarding each bit to the
  next station
• Modify bit as it passes
• e.g. to indicate a packet has been copied (ACK)

27
Ring Transmit State Functions

• Station has data to send
• Repeater has permission (via MAC functionality)
• Station may receive incoming bits during
  transmission
• If ring bit length shorter than packet being
  transmitted
• Pass back to station for checking (ACK)
• There could be more than one packet on ring
• Buffer for retransmission later

28
Ring Bypass State

• Signals propagate past repeater with no delay
  (other than propagation delay)
• Typically there is a one bit delay for an
  active repeater
• Partial solution to reliability problem (see
  later)
• Overall improved performance on ring

29
Ring Media Options

• Twisted pair
• Baseband coax
• Fiber optic
• But not broadband coax
• Would have to receive and transmit on multiple
  channels, asynchronously
• Theoretically possible but not practical

30
Problem Timing Jitter on a Ring

• Clocking included with signal (Differential
  Manchester)
• Clock recovered by repeaters
• To know when to sample signal/recover bits
• Use clocking for retransmission
• Clock recovery frequency deviates from ideal
• Noise imperfections in circuitry
• Retransmission without distortion but with timing
  error
• Cumulative effect is that bit length of the ring
  varies
• Limits number of repeaters on ring
• Solutions
• Use Phase Locked Loop minimize deviation
  between bits
• Use buffers at one or more repeaters Hold a
  certain number of bits to keep bit length of ring
  constant

31
Other Potential Ring Problems

• Break in any link disables network
• Repeater failure disables network
• Installation of new repeater to attach new
  station requires identification of two
  topologically adjacent repeaters
• Method of removing circulating packets required
• With a backup mechanism in case of errors
• Mostly solved with star-ring architecture

32
Star Ring Architecture

• Feed all inter-repeater links to single site
• Concentrator sits at central site
• Provides central access to signal on every link
• Easier to find faults
• Can launch message into ring and see how far it
  gets
• Faulty segment can be disconnected and repaired
  later
• New repeaters can be added easily
• Bypass relay can be moved to concentrator
• Can lead to long cable runs
• Can connect multiple rings using bridges

33
Star Topology

• Each station connected directly to central node
• Usually via two point to point links (one for
  each direction)
• Why is this important in LANs? Phone cabling uses
  star-based wiring
• Central node can broadcast
• Physical star, logical bus
• Only one station can transmit at a time
• Central node can act as frame switch

34
Star LANs

• Typically use unshielded twisted pair wire
  (telephone)
• Potentially lower installation/operations cost
  (use of installed base)
• All stations attach to a central active hub
• All stations have two links to central hub
• One each for transmit and receive
• Hub repeats incoming signal on all outgoing lines
• Link lengths limited to approximately 100m
• Fiber optic cable - up to 500m
• Typically acts as a logical bus - with collisions
• Additional layers of hierarchy are possible

35
Two Level Star LAN Topology
36
Star LAN Hubs and Switches

• Shared medium hub
• Central hub
• Hub retransmits incoming signal to all outgoing
  lines
• Only one station can transmit at a time
• With a 10-Mbps LAN, total capacity is 10-Mbps
• Switched LAN hub
• Hub acts as switch
• Incoming frame switches to appropriate outgoing
  line
• Unused lines can also be used to switch other
  traffic
• With two pairs of lines in use, overall capacity
  is now 20-Mbps per station (full-duplex operation)

37
LAN Switch Characteristics

• No change to software or hardware of devices
• Each device has dedicated capacity
• Scales well
• Two basic types of switches
• Store and forward switch
• Accept input, buffer it briefly, then output
• Cut through switch
• Take advantage of the destination address being
  at the start of the frame
• Begin repeating incoming frame onto output line
  as soon as address recognized
• May propagate some bad frames

38
Hubs and Switches
39
Ethernet IEEE 802.3 in detail

• The most widely used LAN technology today is
  Ethernet and its successive generations and it
  is to this day still evolving!
• Ethernet was originally developed in early 1973
  at the Xerox Palo Alto Research Center (PARC) it
  originally ran at 2.94-Mbps
• The first widespread standard for 10-Mbps
  Ethernet was ratified by DEC, Intel, and Xerox in
  1980 with a major revision in 1983
• The IEEE 802.3 working group developed a
  vendor-neutral standard based on these earlier
  standards (though there are slight differences)

40
802.3 Medium Access Control

• Ethernet and IEEE 802.3 are based on a Medium
  Access Control scheme known as CSMA/CD
• Carrier Sense Multiple Access with Collision
  Detection
• Distributed MAC protocol
• All stations must follow the rules
• Random Access
• Stations access medium randomly based on need to
  transmit data
• Contention
• Stations contend for time on medium to transmit

41
ALOHA

• The predecessor to CSMA/CD developed by the
  University of Hawaii for use in packet radio
  networks
• ALOHA transmission technique
• When station has frame, it sends immediately
• Station listens (for max round trip time) plus
  small increment
• Receiver checks frame check sequence (as in HDLC)
• If frame OK and address matches receiver, send
  ACK
• If no ACK from receiver, sender times out and
  retransmits (up to some number of attempts)
• Frame may be damaged by noise or by another
  station transmitting at the same time (collision)
• Any overlap of frames causes collision
• Has a maximum utilization of (only) 18!

42
Slotted ALOHA

• An optimization to ALOHA developed to improve
  efficiency
• Time divided into uniform slots equal to frame
  transmission time
• Need central clock (or other sync mechanism)
• Transmission must begin at a slot boundary
• Frames either miss or overlap totally
• Max utilization increases to 37 but not as
  simple as regular ALOHA

43
CSMA

• ALOHA does not take advantage of a key
  characteristic of packet radio and local area
  networks!
• Propagation time is much less than transmission
  time
• All stations know almost immediately about
  transmitted frames
• A better technique (CSMA)
• Listen for clear medium (carrier sense)
• If medium is idle then transmit
• If two stations start at the same instant,
  collision
• Wait reasonable time (round trip plus ACK
  contention)
• If no ACK received then retransmit
• Maximum utilization depends on propagation time
  (medium length) and frame length
• Longer frame shorter propagation time gives
  better utilization

44
CSMA behavior under busy conditions

• It would seem reasonable to assume that a station
  should transmit immediately when the medium
  becomes idle
• This is indeed done by CSMA and is called
  1-persistent CSMA
• Minimizes latency for stations
• However, if two or more stations are waiting,
  collisions will occur
• Other options
• Non-persistent CSMA
• N-persistent CSMA

45
CSMA/CD

• With CSMA, collision occupies medium for duration
  of transmission very inefficient!
• Ethernet improves on CSMA by including Collision
  Detection
• Stations listen while transmitting
• If medium idle, transmit immediately
• If busy, listen for idle, then transmit
• If collision detected, transmit brief jamming
  signal
• After jam, wait a random time delay then start
  again
• The random time delay uses a technique called
  binary exponential backoff to maintain network
  stability

46
CSMA/CDOperation
47
Collision Detection

• On baseband bus, collision produces much higher
  signal voltage than signal
• Collision detected if cable signal greater than
  single station signal
• Signal attenuated over distance so signal
  balancing is important
• One consideration that limits Ethernet segment
  distance to 500m (10Base-5) or 200m (10Base-2)
• For twisted pair (star-topology) activity on more
  than one port is collision
• Special collision presence signal

48
IEEE 802.3 Frame Format
49
IEEE 802.3 10-Mbps Physical Layer Specifications

• Special IEEE nomenclature for standards
• ltdata rategtltSignaling methodgtltMax segment lengthgt
• 10Base-5 10Base-2 10Base-T 10Base-FP
• Medium Coaxial Coaxial UTP 850nm fiber
• 0.4 0.25 EIA Cat. 3 62.5/125um
• Signaling Baseband Baseband Baseband Manchester
• Manchester Manchester Manchester On/Off
• Topology Bus Bus Star Star
• Nodes 100 30 - 33

50
100-Mbps IEEE 802.3u

• Also known as Fast Ethernet, first developed by
  Kalpana in 1991-1992
• Soon afterward, the IEEE 802.3 committee
  standardized the Ethernet-compatible 100-Mbps LAN
  technology
• All of the 100BASE-T physical layer options use
  the standard IEEE 802.3 MAC protocol and frame
  format
• IEEE 802.u contains several PHY options
• 100BASE-TX (2 Pair Category 5 UTP or STP)
• 100BASE-FX (1 Pair multi-mode or single-mode
  fiber)
• 100BASE-T4 (4 Pair Category 3 UTP)
• Fast Ethernet also ushered in full-duplex
  operation
• Doubles aggregate speed to and from a station
• CSMA/CD operation is no longer necessary on a
  full-duplex link!

51
Gigabit Ethernet

• Sensing the need for speed, the IEEE 802.3
  committee began work in 1995 on a Gigabit
  Ethernet standard
• Needed as a backbone network for Fast Ethernet
  LANs
• Gigabit Ethernet retains the 802.3 MAC frame
  format, so it is compatible with 10-Mbps and
  100-Mbps Ethernets
• Like IEEE 802.3u, Gig-E has half full duplex
  modes
• Required special modifications for half-duplex
  mode
• Half-duplex equipment operation is rarely seen!
• Gig-E also has a variety of physical layer
  options
• Now 10-Gigabit Ethernet is the high-end, with
  higher speeds under study!

52
Gigabit Ethernet PHY Options

• 1000Base-SX
• Short wavelength (850nm) on multi-mode fiber
  (MMF)
• 1000Base-LX
• Long wavelength (1300nm) on MMF or single mode
  fiber
• 1000Base-CX
• Copper jumpers lt25m, shielded twisted pair
• 1000Base-T
• 4 pairs, Category 5e UTP
• Physical Layer Signaling
• 8B/10B on fiber links PAM-D5 on twisted pair

53

• Wireless Local Area Networks
• (WLANs)

54
Introduction to WLANs

• Why Wireless?
• Mobility
• Flexibility
• Good solution for hard to wire areas
• Why wireless now?
• Reduced cost of wireless systems
• Improved performance of wireless systems

55
Wireless LAN Applications

• LAN Extension
• Extends a wired LAN into spaces
  difficult/impossible to wire (factories, historic
  buildings, etc.)
• Cross building interconnection
• Inter-building interconnect across public
  rights-of-way (alternative to carriers and/or
  wired paths)
• Nomadic access
• Allows mobile users to roam
• Ad hoc networks
• On-the-fly networking

56
Single Cell Wireless LAN (WLAN)
57
Multi Cell Wireless LAN (WLAN)
58
Wireless LAN Configurations
59
General WLAN Requirements

• Throughput
• Number of nodes
• Connection to backbone
• Service area
• Battery power consumption
• Transmission robustness and security
• Collocated network operation
• License free operation
• Handoff/roaming
• Dynamic configuration

60
Wireless LAN Technologies

• Currently three fundamental types of WLAN
  technologies
• Infrared (IR) LANs
• Spread spectrum LANs
• Narrowband microwave

61
Infrared Wireless LANs

• Commonly used in remote control wireless
  printing
• Strengths
• Unlimited unregulated spectrum
• Does not penetrate walls (security, frequency
  reuse)
• Can be reflected by light-colored objects
• Simple transmitters/receivers
• Weaknesses
• Ambient light degrades operation
• Eye Safety power consumption
• Techniques
• Directed-beam
• Omnidirectional
• Diffused

62
Narrowband Microwave WLANs

• Uses just enough spectrum to accommodate the
  necessary transmission bandwidth
• Most common use is point-to-point data links
• Licensed Use
• Traditionally the most widely used variety
• License guarantees interference-free operation
• Must deal with license procedures fees (which
  differ country to country)
• Unlicensed Use
• Systems that use the ISM bands (e.g. 2.4GHz)
• FCC requires low power output

63
Spread Spectrum IEEE 802.11 Wireless LANs

• The IEEE 802.11 set of wireless LAN standards is
  the foundation of almost all current
  installations
• All 802.11 PHY standards are based on spread
  spectrum
• Basic Terminology
• Access Point
• Basic service set (cell) -- BSS
• Set of stations using same MAC protocol (shared
  medium)
• May be isolated
• May connect to backbone via an access point
  (bridge)
• Extended service set -- ESS
• Two or more BSS connected by distributed system
• Appears as single logical LAN to the LLC sublayer

64
Types of Wireless LAN stations

• Standard defines stations based on mobility
• No transition
• Stationary or moves within direct communication
  range of single BSS
• BSS transition
• Moves between BSS within single ESS
• ESS transition
• From a BSS in one ESS to a BSS in another ESS
• Disruption of service likely
• Other protocols proposed for solution (Mobile IP,
  IAPP)

65
Wireless LANs Basic 802.11 Physical Layer
Options

• Infrared
• 1-Mbps and 2-Mbps speeds supported
• Operates at wavelengths between 850-950nm
• Direct sequence spread spectrum
• Operates in 2.4-GHz ISM band
• Up to 7 channels
• 1Mbps or 2Mbps speeds
• High rate option (802.11-b) has speeds up to
  11-Mbps
• Frequency hopping spread spectrum
• 2.4GHz ISM band
• 1Mbps or 2Mbps speeds
• Other options now with speeds over 100-Mbps!

66
Wireless LANs 802.11 High-Rate PHY Options

• 802.11b
• Extension to standard using DSSS at data rates of
  5.5 and 11-Mbps in the 2.4-GHz ISM band
• Uses Complimentary Code keying
• 802.11a
• High-speed option provides data rates up to
  54-Mbps in the 5-GHz ISM band (fallback rates of
  48, 36, 24, 18, 12, 9, 6-Mbps)
• Uses a complex modulation scheme called OFDM
  (similar to DMT)
• 802.11g
• Latest fully approved option provides data rates
  up to 54-Mbps in the 2.4-GHz ISM band
• Reuses OFDM modulation from 802.11a option
• Equipment can be backwards compatible with
  802.11b systems

67
802.11 Media Access Control

• The special demands of high speed wireless LANs
  required development of a new MAC layer
• Distributed wireless foundation MAC (DWFMAC) has
  two different mechanisms
• Distributed coordination function (DCF)
• Fundamental 802.11 MAC
• Uses CSMA
• No collision detection uses acknowledgements
• Point coordination function (PCF)
• Polling by central master (access point)
• Allows for a basic priority mechanism based on
  timing delays (next slide)

68
802.11 MAC Timing
69
802.11 Frame Format (1)
70
802.11 Frame Format (2)

• Frame Control contains frame control type
  information
• Duration/Connection ID
• In data frames used to specify how long the
  channel can be allocated for successful
  transmission of a frame
• In control frames may contain an association
  identifier
• Addresses specific meaning of addresses and the
  number of address fields in the frame depends on
  the frame type
• Uses standard 48-bit IEEE 802 addressing
• Sequence Control contains two subfields used
  for fragmentation reassembly (fragment
  sequence number)
• Frame Body contains either a LLC PDU or MAC
  control information (either 802.1h or RFC 1042
  encapsulation)
• Frame Check Sequence same 32-bit CRC used with
  other IEEE 802 protocols

71
IEEE 802.11 Summary
72
Homework Reading

• Textbook
• Chapters 15 (except 15.4), 16.1, 16.2, and 17
• Homework 5 (due in three weeks)
• Chapter 15 15.1 and 15.4
• Chapter 16 16.6
• Chapter 18 18.1 and 18.4
• Research on the web (brief answers each 2-3
  sentences)
• What is the next generation of Ethernet after
  10-GigE? Where is it in the standards process?
• Can 10-GigE run on copper cable? What types or
  grades?
• What is 802.11n and what speeds does it promise?
• The textbook lists good web sites for research